1. Field of the Invention
This invention relates to microwave filters and in particular to combline) filters.
2. Description of the Related Art
Filters are electronic circuits which allow electronic signals of certain frequencies, called a "passband", to pass through the filter, while blocking or attenuating electronic signals of other frequencies. FIG. 1 illustrates a conventional bandpass filter 100 disclosed by U.S Pat. No. 4,431,977 issued on Feb. 14, 1987 to Sokola et al. Filter 100 includes a block 110 formed from a dielectric material that is selectively plated with a conductive material (i.e. plated with the exception of areas 140). Block 110 includes holes 101-106 which each extend from the top surface to the bottom surface. Holes 101-106 are also plated with the conductive material.
Coupling between the coaxial resonators provided by plated holes 101-106 in FIG. 1 is accomplished by varying the width of the dielectric material between adjacent coaxial resonators. Specifically, the width of the dielectric material between adjacent holes 101-106 is adjusted by the use of slots 110-114. RF signals are capacitively coupled to and from filter 100 in FIG. 1 by means of input and output electrodes 124 and 125 and corresponding input and output connectors 120 and 122. The resonant frequency of the coaxial resonators provided by holes 101-106 is determined primarily by the depth of hole 104, the thickness of block 110 in the direction of hole 104, and the amount of plating removed from the top of filter 100 near hole 104. Tuning of filter 100 is accomplished by the removal of additional ground plating near the top of each plated hole.
Filter 100 is typically fabricated from expensive dielectric materials, such as barium oxide, titanium oxide, or zirconium oxide, thereby significantly increasing manufacturing costs. Moreover, these dielectric materials are physically heavy, thereby rendering filter 100 inappropriate for applications involving a payload, such as in space, where weight is critical. Futhermore, machining dielectric block 110 to a predetermined size and removing the plating to tune filter 100 requires specialized, i.e. costly, equipment and additional labor, thereby further increasing manufacturing costs.
Additionally, the use of a solid dielectric block, such as block 100 disclosed by Sokola et al., exhibits an insertion loss, i.e. how much signal energy is lost as the signal passes through the filter, which varies significantly based on the type of dielectric material used. Specifically, those skilled in the art recognize that the insertion loss of a filter is inversely proportional to the quality factor Q. Thus, the higher the quality factor Q, the lower the insertion loss. The equation below provides the total quality factor Q.sub.Total of filter 100: ##EQU1## where Q.sub.C is the quality factor of the conductive plating and Q.sub.D is the quality factor of the dielectric block 100. A typical filter 100 has a quality factor Q.sub.C equal to 1000. However, quality factor Q.sub.D ranges from 1500 to 8000. Substituting these values into Equation 1 yields a total quality factor Q.sub.Total which ranges from 600 to approximately 888. Although a higher quality factor Q.sub.D of the dielectric increases the total quality factor Q.sub.Total, Equation 1 demonstrates that the presence of any dielectric, irrespective of the value of Q.sub.D, in filter 100 necessarily decreases the total quality factor Q.sub.Total, thereby increasing the insertion loss of filter 100.
Thus, a need arises for a filter fabricated from a low-cost, lightweight material which is easily manufactured and yet provides high performance.